FIG. 1 is a one-line diagram of a typical arrangement of a main ac supply (e.g., power grid) 10, an on-line Uninterruptible Power Supply (UPS) 12, a load 14, and a battery bank 16. The on-line UPS typically contains a controlled rectifier DR1, a dc-to-ac inverter A1, a static bypass switch S1, and isolation transformers T1 and T2 (T2 is typically a nine-winding transformer, with a three-phase primary for S1, a three-phase primary for A1 and a three-phase secondary for the load, T2 is typically a three-winding transformer for single phase). The dc bus 12-1 is designed specifically to support the bank of batteries 16. The bus can provide charging current and also draw current from the batteries in the event of a power outage on the main ac supply 10. A UPS usually contains a static (electronic) bypass switch S1 so that the output of inverter A1 can be switched to the main supply 10 in the event of a fault or high inrush load.
FIG. 2 depicts an example of a conventional system employing an auxiliary generator 18 in combination with a UPS 12. Battery supplied UPSs provide backup power for short periods of time, typically on the order of minutes. The backup time can be substantially increased by adding a generator to the system. This is typically accomplished using an automatic transfer switch S2, at the input of the UPS 12, as shown in FIG. 2. When the main ac power supply 10 fails, the batteries 16 supply power to the UPS. If the power remains down for a predetermined period of time, the automatic transfer switch S2 will start the auxiliary generator 18 and switch the UPS 12 input to the auxiliary generator output through S2.
The use of an auxiliary generator in this manner has some drawbacks. Typically the auxiliary generator remains idle most of the time. Further, when an auxiliary generator is connected to and supplying power to a load (e.g., load 14), the auxiliary generator may not be operating at an efficient power output level. Many times it would be desirable to increase the power output level, allowing the auxiliary generator to supply power to both the load and the grid. However, the solution shown in FIG. 2 does not allow the auxiliary generator to feed power back into the grid.
FIG. 3 depicts an example of a conventional system employing an auxiliary generator 18 in combination with a UPS 12 and with the ability to supply power from the auxiliary generator 18 back into the grid through synchronizing equipment 19. FIG. 3 contains elements similar to those described above with respect to FIG. 2, and their description is omitted for brevity. The system of FIG. 3 includes a feedback switch S3 connected to the auxiliary generator 18. The feedback switch S3 is connected to synchronizing equipment 19, which is connected to the grid.
When the auxiliary generator is connected to the grid, this is referred to as grid parallel mode. The economics of operating the auxiliary generator in grid parallel mode are determined by a number of factors including the real-time price of fuel and electricity, as well as any arrangements the user has with the utility for peak shaving. The synchronizing equipment 19 allows the auxiliary generator to operate in grid parallel mode and may prevent backfeed of power in the event of a grid failure. However, the synchronizing equipment 19 can be expensive.
In view of the above problems, there is a recognized need a for an energy management system that can supply backup power from a generator or from a dc storage device and also output that power back into the grid, without requiring synchronization or costly backfeed prevention equipment.